**3. Synergistic effect in SF6 alternative gas mixtures**

### **3.1 c-C4F8**

c-C4F8 is colorless, odorless, nonflammable, and nonexplosive; the GWP of c-C4F8 is about 8700, and the molecular structure of c-C4F8 is shown in **Figure 11**. The insulation performance of pure c-C4F8 gas is better than SF6, and the gas mixture of c-C4F8 gas has similar insulation characteristics to SF6, which can meet the requirements of practical application. The liquefaction temperature of c-C4F8 is about −6°C, higher than that of SF6, which is −63.8°C [22]. Therefore, c-C4F8 can only be mixed with the gas in a certain proportion to reduce the overall liquefaction temperature of the gas mixture for application.

While studying the synergistic effect of SF6 gas mixture, Christophorou et al. also conducted a comparative study on the synergistic effect of c-C4F8 gas mixtures.

**117**

**Table 1.**

*Synergistic coefficient for c-C4F8/N2.*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

The insulation characteristics and synergistic effect of c-C4F8/CF4 and c-C4F8/CHF3

*Breakdown voltage of c-C4F8 gas mixtures for various electrode gaps. (a) c-C4F8/CF4, (b) c-C4F8/CHF3, (c)* 

It can be seen from the figure that the addition of a small amount of c-C4F8 can greatly improve the insulation strength of gas mixture. Mixing 20% c-C4F8 in CHF3 can double the breakdown voltage. The insulation strength of c-C4F8/1,1,1-CH3CF3 gas with the same mixing ratio is the same as that of pure SF6 gas. By comparison, it can be seen that the synergistic effect phenomenon will also occur when c-C4F8 gas

Xing et al. discussed the feasibility of replacing SF6 gas with c-C4F8/N2 mixture for gas insulation equipment from the perspective of PD performance. The initial PD voltage of c-C4F8/N2 gas mixture was measured under different pressure, different mixing ratio, and different electrode distance. The influence of these three factors on the PD performance of the gas mixture was obtained and compared with the initial PD voltage of pure SF6 gas. The results show that the initial PD voltage of pure c-C4F8 gas is 1.3 times that of pure SF6 gas [23]. c-C4F8 and N2 have synergistic effect, and the synergistic coefficient calculated by Takuma's equation under differ-

Yamamoto et al. examined the insulation characteristics of c-C4F8 gas mixtures such as c-C4F8/N2, c-C4F8/air, and c-C4F8/CO2 under different electric field [24]. The experimental results show that c-C4F8/CO2 gas mixture has better synergistic effect than the other two mixtures, and improve gas pressure or gap distance can greatly

10 0.20 0.48 0.50 0.54 0.53

20 0.20 0.47 0.86 0.45 0.52

30 0.20 0.15 0.25 0.30 0.36

*C* **5% c-C4F8 10% c-C4F8 15% c-C4F8 20% c-C4F8**

0.25 0.20 0.40 0.44 0.63

0.25 0.34 0.50 0.49 0.54

0.25 0.24 0.32 0.34 0.41

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

gas mixture are shown in **Figure 12** [13].

**Electrode distance/mm Gas pressure/**

**Figure 12.**

*c-C4F8/1,1,1-CH3CF3.*

and the gas with large dipole moment are mixed.

ent electrode distance and gas pressure is shown in **Table 1**.

**Mpa**

**Figure 11.** *Molecular structure of c-C4F8.*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

*Modern Applications of Electrostatics and Dielectrics*

**3. Synergistic effect in SF6 alternative gas mixtures**

temperature of the gas mixture for application.

electrodes is shown in **Figure 9** [21].

coefficient *h* of SF6/N2 and *k* under lightning impulse with different needle-plane

The analysis results show that under the action of negative lightning impulse voltage, the negative synergistic effect increases with the increase of gas pressure. The synergistic effect under the positive impact voltage decreases with the decrease of gas pressure. With the increase of the electric field inhomogeneity coefficient, the synergistic effect has a negative synergistic effect. The analysis of the development process of the flow discharge shows that there are three reasons for the negative synergistic effect: similar flow corona starting voltage, different space charge effects, and different N2 and SF6/N2 mixed gas discharge forms. The difference of breakdown voltage between N2 and SF6/N2 gas mixture is shown in **Figure 10**. *r*sp represents the range of space charge, Δ*U*SP is the influence of space charge on breakdown voltage, *U*st is the streamer corona onset voltage, and *U*b = *U*st + Δ*U*SP.

c-C4F8 is colorless, odorless, nonflammable, and nonexplosive; the GWP of c-C4F8 is about 8700, and the molecular structure of c-C4F8 is shown in **Figure 11**. The insulation performance of pure c-C4F8 gas is better than SF6, and the gas mixture of c-C4F8 gas has similar insulation characteristics to SF6, which can meet the requirements of practical application. The liquefaction temperature of c-C4F8 is about −6°C, higher than that of SF6, which is −63.8°C [22]. Therefore, c-C4F8 can only be mixed with the gas in a certain proportion to reduce the overall liquefaction

While studying the synergistic effect of SF6 gas mixture, Christophorou et al. also conducted a comparative study on the synergistic effect of c-C4F8 gas mixtures.

**116**

**Figure 11.**

**3.1 c-C4F8**

*Molecular structure of c-C4F8.*

**Figure 12.** *Breakdown voltage of c-C4F8 gas mixtures for various electrode gaps. (a) c-C4F8/CF4, (b) c-C4F8/CHF3, (c) c-C4F8/1,1,1-CH3CF3.*

The insulation characteristics and synergistic effect of c-C4F8/CF4 and c-C4F8/CHF3 gas mixture are shown in **Figure 12** [13].

It can be seen from the figure that the addition of a small amount of c-C4F8 can greatly improve the insulation strength of gas mixture. Mixing 20% c-C4F8 in CHF3 can double the breakdown voltage. The insulation strength of c-C4F8/1,1,1-CH3CF3 gas with the same mixing ratio is the same as that of pure SF6 gas. By comparison, it can be seen that the synergistic effect phenomenon will also occur when c-C4F8 gas and the gas with large dipole moment are mixed.

Xing et al. discussed the feasibility of replacing SF6 gas with c-C4F8/N2 mixture for gas insulation equipment from the perspective of PD performance. The initial PD voltage of c-C4F8/N2 gas mixture was measured under different pressure, different mixing ratio, and different electrode distance. The influence of these three factors on the PD performance of the gas mixture was obtained and compared with the initial PD voltage of pure SF6 gas. The results show that the initial PD voltage of pure c-C4F8 gas is 1.3 times that of pure SF6 gas [23]. c-C4F8 and N2 have synergistic effect, and the synergistic coefficient calculated by Takuma's equation under different electrode distance and gas pressure is shown in **Table 1**.

Yamamoto et al. examined the insulation characteristics of c-C4F8 gas mixtures such as c-C4F8/N2, c-C4F8/air, and c-C4F8/CO2 under different electric field [24]. The experimental results show that c-C4F8/CO2 gas mixture has better synergistic effect than the other two mixtures, and improve gas pressure or gap distance can greatly


#### **Table 1.** *Synergistic coefficient for c-C4F8/N2.*

**Figure 13.** *Relationship of synergistic effect coefficient* C *with Pd.*

enhance the synergistic effect of c-C4F8 gas mixture. The relationship of synergistic effect coefficient *C* with Pd is shown in **Figure 13**.

#### **3.2 CF3I**

CF3I gas is odorless, nonflammable, chemically stable, and material compatible. The molecular structure of CF3I is shown in **Figure 14**. CF3I is considered as one of the ideal alternatives to conventional Freon refrigerant. In terms of environmental characteristics, CF3I is an extremely environmental friendly gas, and its GWP value is about 1–5, much lower than SF6 gas. At the same time, the C-I chemical bond in the molecular structure of CF3I is easy to photolysis under solar radiation, resulting in the very short existence time of CF3I in the atmosphere, and the ozone destruction potential of the gas can also be ignored. The boiling point of CF3I at normal pressure is −22.5°C, which indicates that CF3I gas will transform from gaseous to liquid when the temperature is lower than −22.5°C [25]. Therefore, CF3I gas must be mixed with buffer gas before it can be used in power equipment.

Urquijo et al. experimentally measured the electron transport parameters of CF3I and CF3I/N2 gas mixture. They analyzed and studied several parameters including electron drift velocity, diffusion coefficient, electron ionization coefficient, and attachment coefficient by means of pulsed Townsend method. Experimental results show that the critical electric field intensity (*E*/*N*)lim of CF3I was 437Td, and the insulation strength of the gas is about 1.2 times that of SF6 gas. When the content of CF3I is 70%, the dielectric strength of CF3I/N2 gas mixture is basically the same with pure SF6 gas [26]. The insulation characteristic comparison of CF3I/N2 and SF6/N2 gas mixtures IS shown in **Figure 15**. From the figure it can be seen that CF3I/N2 and SF6/N2 gas mixture both have synergistic effect phenomenon, but the phenomenon of CF3I/N2 gas mixture is weak when compared with SF6/N2.

In addition to CF3I/N2 mixture, there is also a synergistic effect of CF3I/CO2 mixture. Jiao et al. studied the gas mixture of CF3I/CO2 with low content of CF3I,

**119**

**Figure 14.**

**Figure 15.**

*Molecular structure of CF3I.*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

and conducted breakdown test on CF3I/CO2 gas mixture with different mixing ratio, different gas pressure, and different discharge gap distance under extremely uneven electric field [27]. The experimental results show that the mixing of trace CF3I can significantly increase the breakdown voltage, and the breakdown voltage tends to be stable when the content of CF3I gas is over 6%. Although there is still a gap of the breakdown voltage between CF3I/CO2 and SF6, the trace amount of CF3I has a good synergistic relationship with CO2 mixture. The relationship between breakdown voltage and CF3I content under different electrode distances is shown in **Figure 16**, and the synergistic coefficient of CF3I/CO2 gas mixture calculated by Takuma's equation is shown in **Table 2**. The author believes that the negative synergetic effect coefficient is caused by the saturation of the breakdown voltage and

*Comparison of insulation strength between CF3I/N2 and SF6/N2.*

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

**Figure 14.** *Molecular structure of CF3I.*

*Modern Applications of Electrostatics and Dielectrics*

enhance the synergistic effect of c-C4F8 gas mixture. The relationship of synergistic

CF3I gas is odorless, nonflammable, chemically stable, and material compatible. The molecular structure of CF3I is shown in **Figure 14**. CF3I is considered as one of the ideal alternatives to conventional Freon refrigerant. In terms of environmental characteristics, CF3I is an extremely environmental friendly gas, and its GWP value is about 1–5, much lower than SF6 gas. At the same time, the C-I chemical bond in the molecular structure of CF3I is easy to photolysis under solar radiation, resulting in the very short existence time of CF3I in the atmosphere, and the ozone destruction potential of the gas can also be ignored. The boiling point of CF3I at normal pressure is −22.5°C, which indicates that CF3I gas will transform from gaseous to liquid when the temperature is lower than −22.5°C [25]. Therefore, CF3I gas must be

Urquijo et al. experimentally measured the electron transport parameters of CF3I and CF3I/N2 gas mixture. They analyzed and studied several parameters including electron drift velocity, diffusion coefficient, electron ionization coefficient, and attachment coefficient by means of pulsed Townsend method. Experimental results show that the critical electric field intensity (*E*/*N*)lim of CF3I was 437Td, and the insulation strength of the gas is about 1.2 times that of SF6 gas. When the content of CF3I is 70%, the dielectric strength of CF3I/N2 gas mixture is basically the same with pure SF6 gas [26]. The insulation characteristic comparison of CF3I/N2 and SF6/N2 gas mixtures IS shown in **Figure 15**. From the figure it can be seen that CF3I/N2 and SF6/N2 gas mixture both have synergistic effect phenomenon, but the phenomenon of CF3I/N2 gas mixture is weak when

In addition to CF3I/N2 mixture, there is also a synergistic effect of CF3I/CO2 mixture. Jiao et al. studied the gas mixture of CF3I/CO2 with low content of CF3I,

effect coefficient *C* with Pd is shown in **Figure 13**.

*Relationship of synergistic effect coefficient* C *with Pd.*

mixed with buffer gas before it can be used in power equipment.

**118**

compared with SF6/N2.

**3.2 CF3I**

**Figure 13.**

**Figure 15.** *Comparison of insulation strength between CF3I/N2 and SF6/N2.*

and conducted breakdown test on CF3I/CO2 gas mixture with different mixing ratio, different gas pressure, and different discharge gap distance under extremely uneven electric field [27]. The experimental results show that the mixing of trace CF3I can significantly increase the breakdown voltage, and the breakdown voltage tends to be stable when the content of CF3I gas is over 6%. Although there is still a gap of the breakdown voltage between CF3I/CO2 and SF6, the trace amount of CF3I has a good synergistic relationship with CO2 mixture. The relationship between breakdown voltage and CF3I content under different electrode distances is shown in **Figure 16**, and the synergistic coefficient of CF3I/CO2 gas mixture calculated by Takuma's equation is shown in **Table 2**. The author believes that the negative synergetic effect coefficient is caused by the saturation of the breakdown voltage and

#### **Figure 16.**

*Relationship between breakdown voltage and CF3I content under different electrode distance. (a) 5 mm, (b) 10 mm, (c) 20 mm, (d) 30 mm.*


**121**

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

the abnormal breakdown of the gas mixture containing CO2.

*EEDF of CF3I ternary gas mixtures. (a) CF3I/SF6/N2, (b) CF3I/N2/CF4.*

mixtures is shown in **Figure 17**.

<sup>ξ</sup> = (*E*/*N*) \_\_\_\_\_\_\_\_\_\_\_ lim,mix

coefficient of CF3I ternary gas mixtures are shown in **Figure 18**.

which is −4.7°C [29]. Therefore, it also needs to be mixed with buffer gas.

follows:

**Figure 17.**

**3.3 C4F7N**

does not continue to analyze the reason. In fact, this phenomenon may be related to

In addition to binary CF3I gas mixture, some scholars have studied ternary gas mixture containing CF3I gas. Xu et al. calculated the electron energy distribution function (EEDF) of CF3I ternary gas mixtures by solving the Boltzmann equation and propose a new equation to analyze synergistic effect. The results show that if buffer gases such as N2 or CO2 are concluded in the ternary mixture, the distribution of low-energy electrons in EEDF increases, leading to the synergistic effect, but there is no synergistic effect with CF4. The mixture of two strongly electronegative gases, CF3I/SF6, shows weak negative synergistic effect, while the addition of N2 or CO2 can reduce the negative synergistic effect [28]. The EEDF of CF3I ternary gas

The computational formula of synergistic effect coefficient proposed by Xu is as

− 1 (4)

∑ *xi* (*E*/*N*)lim,i

C4F7N is an SF6 alternative gas jointly developed by Alstom of France and 3 M of the United States. The commodity name of this gas is Novec 4710, which is an organic compound containing four C atoms and seven F atoms. Its molecular structure is shown in **Figure 19**. The chemical characteristics of the gas are similar to those of fluoro organic gas, and the chemical characteristics are relatively stable, which can achieve good compatibility with other materials in electrical equipment. The relative molecular weight of C4F7N is 195.0, and it also has a high liquefaction temperature,

C4F7N gas mixture is a popular alternative to SF6. At present, most researches on SF6 alternative gases are concentrated on this gas. In order to obtain the optimal buffer gas type and mixing ratio in C4F7N gas mixture, Hu et al. studied the power frequency breakdown performance and synergistic characteristics of C4F7N/CO2 and C4F7N/N2 gas mixture with uniform electric field [30]. The gas pressure is

where (*E*/*N*)lim, mix is the critical reduced field intensity of gas mixture and (*E*/*N*)lim, i is the critical reduced field intensity of single gas. *xi* is the molar fraction of the gas component. Fix the buffer mole fraction of gas to 0, 10, and 30%, and change the mixing ratio of CF3I and SF6 gas; the (*E*/*N*)lim and synergistic effect

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

#### **Table 2.**

*Synergetic coefficients of CF3I/CO2 gas mixtures using Takuma's equation.*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

**Figure 17.**

*Modern Applications of Electrostatics and Dielectrics*

*Relationship between breakdown voltage and CF3I content under different electrode distance. (a) 5 mm,* 

**Gap distance /mm Pressure /MPa Synergetic coefficient**

5 0.10 0.11 0.14 0.35 0.72

10 0.10 0.06 0.15 0.13 0.68

20 0.10 0.06 0.15 0.22 0.36

30 0.10 0.08 0.22 0.15 −0.05

*Synergetic coefficients of CF3I/CO2 gas mixtures using Takuma's equation.*

**2%CF3I 4%CF3I 6%CF3I 8%CF3I**

0.15 0.02 0.52 0.22 0.60 0.20 0.06 0.16 0.08 0.99 0.30 0.17 0.10 −0.14 −0.09

0.15 0.06 0.24 0.58 0.27 0.20 0.13 0.12 0.12 0.04 0.30 0.05 0.15 0.02 0.20

0.15 0.21 0.40 0.18 0.31 0.20 0.15 0.16 0.10 0.04 0.30 −0.07 −0.20 −0.82 −1.32

0.15 0.12 0.11 0.19 0.06 0.20 0.04 0.01 0.01 0.38 0.30 −0.05 −0.05 −0.37 −0.32

**120**

**Table 2.**

**Figure 16.**

*(b) 10 mm, (c) 20 mm, (d) 30 mm.*

*EEDF of CF3I ternary gas mixtures. (a) CF3I/SF6/N2, (b) CF3I/N2/CF4.*

does not continue to analyze the reason. In fact, this phenomenon may be related to the abnormal breakdown of the gas mixture containing CO2.

In addition to binary CF3I gas mixture, some scholars have studied ternary gas mixture containing CF3I gas. Xu et al. calculated the electron energy distribution function (EEDF) of CF3I ternary gas mixtures by solving the Boltzmann equation and propose a new equation to analyze synergistic effect. The results show that if buffer gases such as N2 or CO2 are concluded in the ternary mixture, the distribution of low-energy electrons in EEDF increases, leading to the synergistic effect, but there is no synergistic effect with CF4. The mixture of two strongly electronegative gases, CF3I/SF6, shows weak negative synergistic effect, while the addition of N2 or CO2 can reduce the negative synergistic effect [28]. The EEDF of CF3I ternary gas mixtures is shown in **Figure 17**.

The computational formula of synergistic effect coefficient proposed by Xu is as follows:

$$\mathfrak{F} = \frac{\text{(E/N)}\_{\text{lim,mix}}}{\sum\_{l} \text{(E/N)}\_{\text{lim,l}}} - \mathbf{1} \tag{4}$$

where (*E*/*N*)lim, mix is the critical reduced field intensity of gas mixture and (*E*/*N*)lim, i is the critical reduced field intensity of single gas. *xi* is the molar fraction of the gas component. Fix the buffer mole fraction of gas to 0, 10, and 30%, and change the mixing ratio of CF3I and SF6 gas; the (*E*/*N*)lim and synergistic effect coefficient of CF3I ternary gas mixtures are shown in **Figure 18**.

#### **3.3 C4F7N**

C4F7N is an SF6 alternative gas jointly developed by Alstom of France and 3 M of the United States. The commodity name of this gas is Novec 4710, which is an organic compound containing four C atoms and seven F atoms. Its molecular structure is shown in **Figure 19**. The chemical characteristics of the gas are similar to those of fluoro organic gas, and the chemical characteristics are relatively stable, which can achieve good compatibility with other materials in electrical equipment. The relative molecular weight of C4F7N is 195.0, and it also has a high liquefaction temperature, which is −4.7°C [29]. Therefore, it also needs to be mixed with buffer gas.

C4F7N gas mixture is a popular alternative to SF6. At present, most researches on SF6 alternative gases are concentrated on this gas. In order to obtain the optimal buffer gas type and mixing ratio in C4F7N gas mixture, Hu et al. studied the power frequency breakdown performance and synergistic characteristics of C4F7N/CO2 and C4F7N/N2 gas mixture with uniform electric field [30]. The gas pressure is

#### **Figure 18.**

*The (*E*/*N*)lim and synergistic coefficient of CF3I gas mixtures. (a) CF3I/SF6/N2, (b) CF3I/SF6/CO2, (c) CF3I/ CF4/N2.*

**Figure 19.** *Molecular structure of C4F7N.*

0.1–0.7 MPa, and the C4F7N content ratio in gas mixture is 5–20%. The experimental and calculation results show that the C4F7N/CO2 and C4F7N/N2 gas mixture both have synergistic effects, and the synergistic effect of C4F7N/CO2 is stronger than C4F7N/N2. The interaction between C4F7N and CO2 bimolecular is stronger than that of C4F7N and N2, and the research indicates that there is a certain correlation between the synergistic effect of C4F7N gas mixture and the intermolecular interaction. This result presents the theoretical calculation and analysis direction for the qualitative judgment of the synergistic effect of C4F7N gas mixture. The synergistic coefficient h of C4F7N/CO2 and C4F7N/N2 calculated by Guo Can's equation is shown in **Table 3**.

Zheng et al. also studied the synergistic effect and insulation performance of C4F7N/CO2 gas mixture and found that the synergistic coefficient was related to avalanche parameters of pure gas. They proposed a graphical method based on the Wieland approximation to calculate the critical electric field of gas mixture.

**123**

**4. Conclusions**

**Figure 20.**

**Table 3.**

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

**Gas type Pressure /MPa Coefficient** *h*

C4F7N/CO2 0.1 0.57 0.63 0.69 0.77

C4F7N/N2 0.1 0.40 0.46 0.53 0.62

**5% C4F7N 7% C4F7N 9% C4F7N 13% C4F7N**

0.2 0.49 0.59 0.63 0.69 0.3 0.46 0.52 0.57 0.69 0.4 0.45 0.53 0.57 0.69 0.5 0.45 0.54 0.59 0.73 0.6 0.47 0.55 0.62 0.75 0.7 0.50 0.58 0.64 0.76

0.2 0.44 0.51 0.57 0.63 0.3 0.34 0.41 0.48 0.56 0.4 0.35 0.42 0.48 0.56 0.5 0.37 0.43 0.48 0.56 0.6 0.39 0.46 0.50 0.64 0.7 0.44 0.52 0.58 0.64

Based on the formula, the C4F7N content takes 9% in gas mixture tends to be the optimal mixing ratio [31]. **Figure 20** shows the relationship of the critical electric

*The relationship of the critical electric field of gas mixture and C4F7N content.*

With the development of the power system, gas-insulated equipment such as GIS will be more and more widely used. At present, SF6 gas is still the most used insulating gas in power systems. However, due to the greenhouse effect of SF6 gas and the

field of gas mixture and C4F7N content.

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

*Synergistic coefficient* h *of C4F7N/CO2 and C4F7N/N2 gas mixture.*


#### *Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

#### **Table 3.**

*Modern Applications of Electrostatics and Dielectrics*

**122**

**Figure 19.**

**Figure 18.**

*CF4/N2.*

*Molecular structure of C4F7N.*

tion is shown in **Table 3**.

0.1–0.7 MPa, and the C4F7N content ratio in gas mixture is 5–20%. The experimental and calculation results show that the C4F7N/CO2 and C4F7N/N2 gas mixture both have synergistic effects, and the synergistic effect of C4F7N/CO2 is stronger than C4F7N/N2. The interaction between C4F7N and CO2 bimolecular is stronger than that of C4F7N and N2, and the research indicates that there is a certain correlation between the synergistic effect of C4F7N gas mixture and the intermolecular interaction. This result presents the theoretical calculation and analysis direction for the qualitative judgment of the synergistic effect of C4F7N gas mixture. The synergistic coefficient h of C4F7N/CO2 and C4F7N/N2 calculated by Guo Can's equa-

*The (*E*/*N*)lim and synergistic coefficient of CF3I gas mixtures. (a) CF3I/SF6/N2, (b) CF3I/SF6/CO2, (c) CF3I/*

Zheng et al. also studied the synergistic effect and insulation performance of C4F7N/CO2 gas mixture and found that the synergistic coefficient was related to avalanche parameters of pure gas. They proposed a graphical method based on the Wieland approximation to calculate the critical electric field of gas mixture.

*Synergistic coefficient* h *of C4F7N/CO2 and C4F7N/N2 gas mixture.*

**Figure 20.** *The relationship of the critical electric field of gas mixture and C4F7N content.*

Based on the formula, the C4F7N content takes 9% in gas mixture tends to be the optimal mixing ratio [31]. **Figure 20** shows the relationship of the critical electric field of gas mixture and C4F7N content.

### **4. Conclusions**

With the development of the power system, gas-insulated equipment such as GIS will be more and more widely used. At present, SF6 gas is still the most used insulating gas in power systems. However, due to the greenhouse effect of SF6 gas and the

consensus of countries on the development of low-carbon clean energy systems, it is imperative to use new environmental friendly insulating gas in power equipment. Looking for SF6 replacement gas will also be a continuing research hotspot in the field of electrical engineering. According to the current research status, the replacement of SF6 by single gas and its application in gas insulation equipment is still in the laboratory research stage. The use of an electronegative gas mixed with a buffer gas is a well-feasible solution. This paper reviews the research status of the synergistic effect of insulating gas mixtures including SF6 gas. It has been found that some achievements have been made in the process and mechanism of synergistic effect, the types of synergistic effects under different conditions, and their influencing factors:

